It's by and large mathematical software, like all renderers. So it isn't interactive in a manner like software that allows moving a character model and sequencing frames to make an animation. It's a kind of a 'kernel' in some sense for animation and 3D modelling software.
The source files contain the algorithms/computations needed to solve various equations that people involved in Computer Graphics research have came up with to simulate various physical - optical phenomena (lighting, shadows, water reflections, smoke, waves) in the most efficient (fast) and and usually photorealistic sense for a single image (static scene) already created (character/landspace models, textures) in a program.
Since there are various different techniques for the simulation of one specific phenomenon, it's interesting to peek into the tricks used by a very large animation studio.
The source files contain the algorithms/computations needed to solve various equations that people involved in Computer Graphics research have came up with to simulate various physical - optical phenomena (lighting, shadows, water reflections, smoke, waves) in the most efficient (fast) and and usually photorealistic sense for a single image (static scene) already created (character/landspace models, textures) in a program.
Since there are various different techniques for the simulation of one specific phenomenon, it's interesting to peek into the tricks used by a very large animation studio.